Transitional cell and cell cancer of the bladder was induced in Wistar/Lewis female rats by direct instillation of N-methyl-N-nitrosourea into the bladder. Feeding of the synthetic retinoids, 13-cis-retinoic acid, inhibited the incidence and extent of bladder cancer in these rats, even when 13-cis-retinoic acid administration was begun after completion of the carcinogen treatment. BIBLIOGRAPHIC REFERENCES: Sporn, M.B., Squire, R.A., Brown, C.C., Smith, J.M., Wenk, M.L., and Springer, S. 13-cis-retinoic acid: Inhibition of bladder carcinogenesis in the rat. Science 195: 487-489, 1977.